Chemical Plant spacing

8/7/2019 Chemical Plant spacing

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September 1975

Revised December 1998

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SPACING OF FACILITIES IN OUTDOOR CHEMICAL PLANTS

Table of ContentsPage

FLAMMABLE VAPOR OR GAS EXPLOSIONS IN THE OPEN .................................................................. 2

FLAMMABLE VAPOR OR GAS EXPLOSIONS IN PROCESS EQUIPMENT ............................................ 2

OTHER VESSEL FAILURES ........................................................................................................................ 2

DETONATIONS ............................................................................................................................................. 3

DAMAGE FROM BLAST WAVES ................................................................................................................ 3

SPACING OF FACILITIES ............................................................................................................................ 4

SEPARATION GUIDELINES ......................................................................................................................... 5

List of Figures

Fig. 1A. Spacing distances for outdoor chemical processing equipment. (English units) ............................ 3Fig. 1B. Spacing distances for outdoor chemical processing equipment. (Metric units) .............................. 4

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1975 Factory Mutual Engineering Corp. All rights reserved. No part of this document may be reproduced, stored in a retrieval system,

or transmitted, in whole or in part, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without writtenpermission of Factory Mutual Engineering Corp.


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DETONATIONS

A detonationis a propagating reaction which progresses at or above sonic velocity in the reactant. The shock

wave produced is somewhat similar to that produced by commercial explosives or by catastrophic ruptureof a vessel. If the energy can be predicted in terms of TNT equivalent, the extent of damage to structures or

equipment of varying types of construction can be predicted with reasonable accuracy.

Sometimes detonable materials such as acetylides or organic peroxides are accidently produced or accumu-

lated. If these can be anticipated and predicted, it may be possible to provide safe isolation for such haz-

ardous processes.

DAMAGE FROM BLAST WAVES

Studies indicate that the characteristics of the blast waves from vessel ruptures and from detonations are

not identical. However, they are so similar that a common set of curves can be derived which agrees rea-

sonably well with theoretical calculations, experimental tests, and loss experience. These curves are Fig. 1A

(for English units), and Fig. 1B (for metric units).

Using the curves, the energy released by expanding gas liberated by catastrophic rupture of a vessel can be

approximately related to the equivalent weight of TNT. This is done by using the volume of the vessel andthe failure pressure of the vessel. As an estimate the vessel failure pressure may be taken as four times the

design pressure. If a deflagration is confined to the vapor phase and the liquid in the vessel is below its atmo-

spheric boiling point, use only the volume of the vapor phase under the worst credible condition. If the liq-

uid is heated more than 50F (28C) above its atmospheric boiling point so that a substantial amount of the

liquid will turn to vapor on vessel failure, the failure pressure should be increased by a percentage equal

to the percent of liquid in the vessel, and the full vessel volume should be used.

For example, a 2,000-gallon (7.5-m3) reactor contains 1,000 gallons (3.8 m3) of liquid and is subject to a ther-

mal explosion with failure expected at 150 psig (1000 kPa) (10 bars). Since the reactor will be 50% full of

an overheated liquid, the effective pressure at failure will be 150 psig 150% = 225 psig (1500 kPa) (15 bars).

With a reactor volume of 2,000 gallons (7.5 m3) from Fig. 1, the TNT equivalent will be about 15 lb (6 kg).

Fig. 1A. Spacing distances for outdoor chemical processing equipment. (English units)

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1975 Factory Mutual Engineering Corp. All rights reserved.


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Once the TNT equivalent is obtained, it is possible to estimate the approximate radius of severe damage using

the same curves. Severe blast damage normally occurs over a greater distance than missile damage. Chemi-

cal process equipment can normally withstand substantial blast pressures but is more subject to missile dam-

age. Spacing in accordance with that for light wall sheathing is recommended to minimize missile damage.In the above example where a yield of 15 lb (6 kg) of TNT is expected, a spacing of about 110 ft (33 m)

is specified for light sheathing and framing, load-bearing masonry construction, and process vessels. Exten-

sive damage to large windows may be expected for about 220 ft (66 m). Such window damage may be

acceptable unless vital facilities inside such as computer or control equipment could be damaged by glass

fragments or the blast wave entering the room. Beyond 220 ft (66 m) some windows may be broken, but inte-

rior damage should be light. Wherever there is an explosion hazard, damage from missiles, vapor cloud

explosions, and ensuing fires is expected within 100 ft (30 m). Greater spacing for fire exposure only may

be needed as outlined in applicable Factory Mutual recommended practices.

Some materials used in chemical processes may be subject to detonation. These include some organic per-

oxides, acetylenic compounds, commercial explosives, and compounds of similar instability. If the quantity

and type of material present is known and the TNT equivalent can be obtained, Fig. 1Aor Fig. 1B may be used

to determine the extent of expected damage.

SPACING OF FACILITIES

Separation or other protection is normally provided between an exposed facility which must be protected

and a hazard which may constitute a fire or blast exposure, or both. Many facilities expose one another

because they constitute both high values and a severe hazard.

Sufficient spacing between major process blocks, tank farms, important buildings, and major plant utilities

is normally needed to confine fire and explosion damage to the major unit of origin.

Equipment and structures within a process block are usually spaced in accordance with production require-

ments. It is seldom practical to isolate such individual facilities except important control houses and unusu-

ally hazardous process equipment.

Fig. 1B. Spacing distances for outdoor chemical processing equipment. (Metric units)

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Separation or other physical protection is normally needed to protect the following from an exposing fire or

explosion:

1. Process equipment of high value or importance to production, within buildings or outside.

2. High value storage facilities, such as large tanks, tank farms, warehouses, or yard storage.

3. Utility buildings and equipment vital to production or fire protection, such as major boilers, compressors,

water and electrical services, pump houses, cooling towers, and maintenance shops.

4. Important control rooms and administrative buildings.

The following can be sources of fire exposure and need separation from high value processes.

1. Large quantities of flammable liquids or gases in a tank, tank farm, process area, processing building,

drainage area, flare stacks, or burning pits.

2. Large quantities of ordinary combustibles in a building or in yard storage.

The following can be the source of a flammable gas or vapor cloud, which can explode in the open or enter

other buildings: processes or storage facilities at such temperatures and pressures that release of large quan-tities of flammable vapors or gases can be expected upon failure of equipment.

The following can be sources of blast or missile damage due to rupture of process vessels or detonations,

and need separation from high value processes:

1. Processes or storage containing unstable or explosive materials.

2. Process reactions of an exothermic nature where, because of newness, lack of complete investigation,

or nature of the reaction itself, a strong possibility of the reaction getting out of control exists. Included would

be some nitration or polymerization reactions.

3. Processes using equipment of large volume which must be taken through the explosive range at peri-

odic intervals for maintenance purposes.

4. Processes which have a significant history of explosions.

5. Unprotected process or storage tanks under pressure where weakening of the pressurized shell can resultfrom an external exposure fire.

6. Processes at pressures over 5000 psi (34.5 MPa) (345 bars).

Many facilities will fall into more than one group. Separation needed is the greatest distance, based on the

most severe combination.

SEPARATION GUIDELINES

The following guidelines are for fire separation and predictable explosion separation for the planning of new

plants, additions to existing plants, or the evaluation of existing plants. Vapor cloud explosions in the open

can cause damage in excess of practical separation distances.

Judgement must be used to determine the extent of damage which can be expected, and the separation

or other protection needed. It may not be possible to rearrange existing plants to provide the needed spac-

ing, but changes made over the years should progress toward a satisfactory level of safety. Unusually severehazards should be minimized through extraordinary preventive procedures such as blast resistant barri-

cades, until adequate isolation can be provided.

1. Separation between a high value facility and fire exposure:

Distances for fire exposure only should be based on applicable Factory Mutual recommended practices for

exposure protection, flammable liquid storage tanks, yard storage, etc. For flammable liquids, drainage facili-

ties, dikes, and the slope of land should be taken into consideration.

2. Separation between a high value facility and a vapor cloud exposure:

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Because of the unpredictibility of the area which may be affected by a release of flammable vapors or gases

in the open, a separation distance beyond that required for fire exposure in guideline No. 1 or blast expo-

sure in guideline No. 3 is seldom practical. To minimize the possibility of entry of vapors into buildings, the dis-

tance should be not less than 100 ft (30 m); more, if ground slopes so that flow is toward the building, orif discharge of vapors is very likely as from flare stacks or normally venting relief valves.

3. Separation between a high value facility and a blast or missile exposure:

a. For materials such as LP gas, ethylene oxide, hydrogen peroxide, organic peroxides, commercial explo-

sives, ammonium nitrate, etc., for which Factory Mutual loss prevention practices are provided, use the

specific spacing recommendations contained therein.

b. For other blast or missile hazards, use spacing guide lines in Figures 1 and 2, with a minimum of 100

ft (30 m). Where needed distances cannot be provided, the exposing hazard should be barricaded by

blast and missile resistant construction or the exposed facility should be of blast and missile resistant con-

struction. (Design data is available in Structures to Resist the Effects of Accidental Explosions, Depart-

ments of the Army, Navy and Air Force Manual (TM 5-1300, NAVFAC P-397, AFM 88-22) November 1990

Edition.)

There is no NFPA standard on this subject.

FMELPC June 1975

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Chemical Plant spacing